Emily Smyk's Honours Thesis Abstract

Thesis Title: 
Geochemistry and Petrography Study of a Mesoarchean Felsic Metavolcanic Unit Near Musselwhite Mine, North Caribou Greenstone Belt

The ~3 Ga North Caribou greenstone belt in the North Caribou Terrane, Superior Province, comprises greenschist- to upper amphibolite-facies, ultramafic to felsic metavolcanic and metasedimentary rocks, intruded by 3000 to 2700 Ma felsic plutonic rocks. The study area was centered on a narrow (~100m wide), north-trending, felsic metavolcanic unit on the western side of Opapimiskan Lake, 5 km northwest of Musselwhite Mine, in the South Rim Volcanic Unit (SRV).  The felsic metavolcanic unit was mapped in detail over a distance of 2 km. Samples of the felsic rocks, as well as the associated metabasalts, metasedimentary, and intrusive units were collected and classified according to their petrography and whole rock geochemistry.

The metarhyolitic units are defined by mineral assemblage of quartz + plagioclase + K-feldspar with accessory muscovite + biotite + chlorite ± titanite ± clinozoisite ± zircon. Field observations identified some flows, tuffaceous and pyroclastic units as well as felsic dykes related to regional plutonism. The calculated anhydrous SiO2 values for the felsic metavolcanic rocks in the SRV range from 75 – 81 wt%. These high-silica, calc-alkaline metarhyolites are classified as FII-type rhyolites. The unaltered felsic metavolcanic rocks are LREE-enriched with εNd values of -2.79 and -0.89.

The tholeiitic metabasaltic and komatiitic-basalt units have been metamorphosed to amphibolites composed almost entirely of plagioclase and hornblende with accessory titanite + chlorite + opaques ± rutile ± biotite ± zircon. The komatiitic-basalt has a MgO value of 14 wt% (compared to 3 to 8 wt% MgO for the metabasalts) and has greater Ni and Cr (ppm) contents. The basalts can be subdivided into two classes based on their REE contents, those with flat REE and those that are LREE-enriched with negative Nb and Al anomalies. The εNd values are +0.78 and +0.58 for the flat REE basalts and -1.58, -3.35, and -1.61 for the LREE-enriched basalts.

Metasedimentary rocks in the study area have been metamorphosed into schists and a quartzite. The protolith of the schists were probably pelitic sedimentary rocks. The mineral assemblage consists of strongly foliated quartz + biotite + muscovite + chlorite ± garnet ± zircon ± clinozoisite. These schists are associated with the felsic metavolcanic rocks. The quartzite is similar to the felsic metavolcanic units but does not contain feldspars. Its mineral assemblage comprises quartz + muscovite + biotite + garnet.

The metarhyolites plot as calc-alkaline rocks with consistent LREE enrichment with Nb and Ti depletions. This trend (including the LREE-enriched metabasalts and komatiitic-basalt) suggests a bimodal volcanic arc tectonic environment. The relative enrichment of LREEs occurs as the LILE enriched fluids are driven off the downgoing slab and modify the overlying mantle wedge. The negative Nb anomalies and negative εNd value suggests that the melt interacted with and assimilated older continental crust. The tholeiitic metabasalts with flat REE patterns suggest an oceanic island plateau environment generated by a mantle plume. The positive εNd values of these rocks suggest that the melts were derived from a depleted mantle source possibly a mix of upper depleted mantle and plume material. These geochemical signatures are seen in both the South Rim and Opapimiskan-Markop metavolcanic units. The interpretation of a plume-sourced oceanic island plateau arriving at a continental arc subduction zone validates the models of early proto-continental crust development of plume-sourced crustal material being accreted through subduction processes in the Superior Province.